A display using liquid crystal elements has been applied to various devices such as OA equipment, information terminals, watches and television sets due to having the characteristics of lightweight, thinness and low power consumption. Particularly, a liquid crystal element using a thin film transistor (TFT element) has been used, due to high responsiveness, for data display monitors of portable televisions, computers and the like containing a great amount of information.
Recently, with increases in the amounts of information and moving picture displays, high definition, high-speed and wide viewing angle characteristics have been demanded, and the structure of a TFT array has been made minute in accordance with the high fineness.
For high-speed, for example, an OCB mode, a VAN mode, a HAN mode and a π-sequence mode, modes using nematic liquid crystals, a surface stability type ferroelectric liquid crystal (SSFLC) mode using smectic liquid crystals, an anti-ferroelectric liquid crystal (AFLC) mode and the like, have been considered.
Further, wide viewing angle characteristics have been improved by devising liquid crystal driving methods of an IPS mode, VAN mode and OCB mode.
In particular, the VAN mode has recently gained attention since it can obtain a response speed higher than that of a conventional twist nematic (TN) mode and a rubbing orientation treatment process can be removed by adopting vertical orientation treatment, the process conventionally raising serious concerns over occurrence of troubles such as electrostatic discharge failure. Further, since compensation design of a view angle is relatively easy in the VAN mode, the VAN mode can realize a wide viewing angle as a multi-domain type VAN mode (hereinafter, referred to as MVA mode).
As a means for orientation division in the MVA mode, as disclosed in, for example, Japanese Laid-Open Patent Publication No. 2005-292515, there is known a constitution of rib-shaped constructions (projections), lacking portions (slits) of a pixel electrode, or the like which are formed on one or both substrate(s). In the case where a nematic liquid crystal material indicating negative dielectric anisotropy is used, an electric field is tilted to the inside of the slit, and liquid crystal molecules, that is, directors, are tilted to the outside of the slit, by an electric field discrete effect at the slit of the pixel electrode. On the other hand, the directors are tilted to the outside of the projection by a shape effect of the projection on a counter substrate side. There upon, by combining both substrates with each other so that the tilting directions of the directors correspond to each other, orientation division can be excellently performed. Additionally, by making a slit pattern and a projection pattern have anisotropy in a plurality of different directions, for example, forming the slit on the array substrate side and the projection on the counter substrate side so that they respectively shift from the substrate end sides at an approximately 45° angle and shift from each other at approximately 90° so as to have anisotropy of four different directions together, a liquid crystal layer can also be divided into a plurality of domains.
Thus the MVA mode is excellent as a display having a high contrast ratio and a wide viewing angle, however demands have been made for further improvements in the contrast ratio.
As a method for improving the contrast ratio of the MVA mode, it is preferable that an orientation regulating unit does not have a rib-shaped projection structure but a slit structure, and further preferable the area thereof is reduced to the extent possible, because it is important to lower the luminance in black display for obtaining the high contrast ratio. That is, since the directors align perpendicularly to a substrate surface and a phase difference, birefringence index Δn, is 0 in the MVA mode, the luminance of transmitted light in the black display approximates to 0 and the high contrast ratio is obtained in a liquid crystal display in which polarizing plates are bonded with the array substrate and counter substrate respectively so that transmission axes of the plates become orthogonal to each other. However, since the directors located above each of the rib-shaped projections align perpendicularly to a slope of the projection, obliquely, the birefringence index Δn does not become 0, and light slightly leaks. On the other hand, since there is no projection in the slit structure of the pixel electrode, the high contrast ratio is obtained.
However, the director orientation regulating force of the slits of a pixel electrode is smaller than that of the rib-shaped projections, and the force becomes smaller as the area of the slit becomes smaller. Therefore, for example, in the case where a surface of the display is pressed by fingers in white display, the orientation of the directors is easily disturbed. Accordingly, in the case where the slit structure of the pixel electrode is adopted, the number of slits must be increased.
Additionally, as another problem in the case where the slit structure is adopted for a liquid crystal display element of a linear polarization mode, decrease in transmittance is cited. Birefringence of liquid crystal is used for transmission control of light in the MVA mode. Where I represents the transmitted light intensity, I0 represents the incident light intensity, θ represents an angle between the transmission axis (absorption axis) of the polarizing plate and the director, Δn represents a birefringence index of the liquid crystal, d represents the thickness of the liquid crystal layer, and λ represents the wavelength of light, the transmitted light intensity I is calculated by the following expression (1):I=I0·sin22θ sin2(Δn·d·π/λ)  (1)
As indicated by the expression (1), when the angle between the director of the liquid crystal and the transmission axis of the polarizing plate is 45° and the falling angle is large, a ratio of the transmitted light intensity I to the incident light intensity I0, transmittance, becomes maximum. In the case of the slit structure, the falling angle is controlled by the electric field discrete effect, and the angle between the director and the transmission axis of the polarizing plate depends on the width of the slit. That is, in the case where the width of the slit is large, the director falls approximately orthogonally to a longitudinal side of the slit, and in the case where the width thereof is small, the director falls approximately parallel with the longitudinal side of the slit. Additionally, in the case where the width of the slit is large, the center of the slit does not sufficiently obtain the electric field discrete effect, the falling angle of the director becomes small, and the transmittance lowers. Accordingly, it is preferable that the width of the slit is small, and particularly 5 μm or less.
However, even if the width of the slit is thus reduced, the angle between the director of the liquid crystal and the transmission axis of the polarizing plate deviates from 45° indicating the maximum transmittance due to the change in the electric field discrete effect, and the transmittance lowers.
In view of the problem, the present invention has been made and aims at providing a liquid crystal display element for securing the transmittance and contrast ratio and obtaining an excellent wide viewing angle range.